A cryosurgical instrument disclosed in U.S. Pat. No. 4,269,390 employing a standard, double walled, evacuated, metal vacuum bottle or dewar, has a collar metallurgically bonded to the top of the dewar near the mouth thereof to provide machine threads for releasably engaging the delivery and control portion of the instrument, which is mounted on a cap having internal threads, to the dewar of the instrument. The cap has a valve mounted thereon which controls the flow of nitrogen from a feed tube in the dewar to a nozzle mounted on a delivery tube. The valve is opened by a valve operating lever. The cryosurgical instrument of said patent has been in service around the world, with minor modifications, since 1976.
In order to avoid explosive splashing of cryogenic liquid when the cap is removed from the dewar of such an instrument, it has been common in the past to lift the cap of a pressure relief valve, thereby allowing gaseous cryogen to escape, relieving the pressure within the dewar. With the advent of more streamlined pressure relief valves, however, the movement of the pressure relief valve to relieve pressure within the dewar became difficult, and created the risk of undesirable freezing of the skin as a consequence of cold cryogenic gas emanating from the valve. In an attempt to overcome this problem, the threads of the cap have been machined away at one point, so as to provide a passage for the escape of gas once the cap was loosened sufficiently to allow gas to pass by a gasket, such as one-quarter or one-half of a turn. However, it was found that this expedient worked sometimes, but more frequently did not work, so that the operator frequently had to revert to raising of the pressure relief valve in order to release the pressure in the dewar.